The present invention relates to the management of non-volatile memories and, more particularly, to a method of managing a memory such as a flash memory whose physical representation of the data stored therein tends to drift over time.
FIG. 1 illustrates the storage of a bit, either a zero bit or a one bit, in a cell of an electrically programmable memory (EPROM) such as a flash memory. For historical reasons, this process of storing data in a EPROM is called “programming” the EPROM. Specifically, the cell that is the subject of FIG. 1 stores one bit of data, and so commonly is called a single-level cell (SLC). Initially, the cell has a nominal threshold voltage V1 that represents a one bit. For example, after a block of a flash memory has been erased, all the cells have nominal threshold voltages V1. Because of unavoidable inaccuracies in the initializations of the cells, the actual threshold voltages are distributed around the nominal threshold voltage V1 according to a distribution curve 10. Then, to each cell that is to store a zero bit, a train of programming voltage pulses is applied, in order to inject electrons from the cell's silicon substrate through the cell's oxide layer into the cell's floating gate, until the cell's threshold voltage exceeds a reference “program verify” threshold voltage V0 that represents a zero bit. Because the electrons move through the oxide layer by quantum mechanical tunneling or by hot injection, because of non-uniformities in the cells' structures, and because the initial threshold voltages are distributed according to distribution curve 10, the threshold voltages of the cells that store zero bits are distributed above V0 according to a distribution curve 12.
A cell is read by comparing the cell's threshold voltage to a reference “read” threshold voltage VR that is above distribution curve 10 but below V0. If the cell's threshold voltage is below VR then the cell's contents are read as a one bit. If the cell's threshold voltage is at or above VR then the cell's contents are read as a zero bit.
Several factors influence the accuracy with which flash memory cells are read and hence the reliability of data storage by the flash memory cells. The two most important ones are data retention time and program/erase (P/E) cycle history: